Célébration de l'herbe de mer

Les requins à tête blanche ingèrent de grandes quantités d'herbes marines lorsqu'ils se nourrissent de crabes et retiennent les nutriments des plantes. Photo de Shane Gross

Pourquoi la protection de cette plante marine en voie de disparition est essentielle dans notre lutte pour sauver le climat et la biodiversité.

J'ai soulevé la pierre et ma clé avait disparu. Il était midi en juillet, et la chaleur des Keys de Floride était oppressante. Je dégoulinais d'eau salée et j'avais des algues dans les cheveux. Mon téléphone portable, mon portefeuille, mon eau potable, la clé de l'hôtel, mes lunettes de soleil et tout le reste, à l'exception de mon short de surf et de mon appareil photo sous-marin, étaient enfermés dans la voiture de location surchauffée. Je savais que j'avais mis le porte-clés là, et j'avais surveillé la zone depuis ma place dans l'eau. Personne n'aurait pu la prendre, et je me demandais avec incrédulité où elle pouvait bien se trouver.

I had been sitting in a seagrass meadow, hoping a shy bonnethead shark would swim in front of my lens. These small hammerheads were reclassified in 2018 as the world’s first known omnivorous sharks. A central question facing scientists was how much of the seagrass nutrient content the sharks were taking in.

In one study led by Samantha Leigh, Ph.D., of California State University, Dominguez Hills, researchers fed captive bonnethead sharks a 90 percent seagrass diet for three weeks. Analysis of the sharks’ digestion showed they could get nutrients from the seagrass and exhibit growth on a primarily vegetarian diet. Since other marine omnivores have had positive effects on food webs, the researchers posited that the sharks’ nutrient intake meant we would “need to reevaluate the roles of bonnethead sharks in seagrass ecology because they could be responsible for significant grazing and nutrient transport within fragile seagrass ecosystems.”1

Un banc de poissons se nourrit d'algues marines.
Un banc de poissons se nourrit d'herbes marines.

Les requins mangeurs de plantes ne sont qu'un exemple de la façon dont la science change notre vision des herbiers marins et révèle leur importance.

Seagrasses are different from algae and seaweed; they are more akin to flowering plants that grow on land. About 72 species are found in coastal seas on every continent except Antarctica, stretching from the equator to cool and cold areas — some Zostera marina pousse dans l'Arctique et peut survivre sous la glace.

A few weeks before I ended up keyless in the Florida heat, I was wearing a drysuit but still shivering in 35°F (1.7°C) water in Newfoundland, Canada. I had joined a small team of scientists from Fisheries and Oceans Canada (DFO) who have been monitoring the eelgrass and fish populations for decades.

In the early 1990s one of the world’s largest fisheries collapsed. A 1992 moratorium on cod fishing cost more than 30,000 jobs in Newfoundland. The Atlantic cod population had been mismanaged and was severely overfished. In the nearly 30 years since then, the population hasn’t returned to levels where commercial fishing can operate as it once did. Having ample seagrass habitats in which juveniles can hide and feed is one key to increasing cod numbers.

tortue verte
Une tortue de mer verte rencontre par hasard un chercheur d'herbes marines aux Bahamas.

Many of the world’s major fisheries species depend on seagrass at some point in their life cycle. About 70 percent of the fishery species in Florida, for example, spend some time in seagrass.2

Comme les bonnets, les cabillauds de l'Atlantique sont timides avec les gens. J'ai installé mon appareil photo sous-marin sur un trépied devant une zone luxuriante d'herbes marines et j'ai reculé, tandis que ma main gelée saisissait le déclencheur à distance.

A tiny Atlantic cod would occasionally flit in front of my lens, and I’d snap a picture. I joked with the team between dives and sips of hot chocolate how I was on my way to sunny Spain after this, simultaneously attempting to warm them up and make them jealous.

Off the Spanish coast of the Mediterranean Sea, I swam among one of the oldest and largest seagrass species on the planet — Posidonia oceanica. The plant’s root system is connected beneath the seafloor and stretches for more than 9 miles (14.5 km). Scientists estimate this organism could be up to 200,000 years old. The species is slow to recover from loss and has declined due to climate change and other human activity. Researchers are gathering data on its loss and areas where it is not declining, hoping that a better understanding will help reverse the trend.

Cicatrices d'hélice dans les Keys de Floride
Les cicatrices d'hélice, comme celles-ci dans les Keys de Floride, peuvent couper les racines et infliger des dommages à long terme à la prairie.

Les herbiers marins sont en déclin dans le monde entier. Certaines estimations font état d'une perte globale allant jusqu'à 7 % par an au cours des années 1990 et au début des années 2000.3 Des facteurs tels que la pollution, le développement côtier, la diminution de la clarté de l'eau, certaines pratiques de pêche, d'amarrage et de navigation, et, bien sûr, le changement climatique, sont parmi ceux qui contribuent au déclin.

In May 2019 I joined a team of volunteers and researchers from the Virginia Institute of Marine Science (VIMS) as they continued a 20-year seagrass restoration project using a technique developed by VIMS professor Robert “JJ” Orth, Ph.D. His seeding system is incredibly effective and has reestablished more than 9,000 acres of healthy eelgrass that has shown signs of developing into a full ecosystem. This method may be promising for regrowth elsewhere, but protecting what we still have should be the priority.

On ne saurait trop insister sur les avantages de la protection des herbiers marins. Les herbiers marins purifient l'eau, contribuent à la protection contre l'érosion côtière, aident à maintenir les pêcheries à petite échelle qui soutiennent les communautés, et augmentent les populations de poissons et la biodiversité. Elle séquestre beaucoup plus de carbone par surface que les forêts terrestres et réduit l'acidification des océans. Des herbiers marins en bonne santé sont synonymes d'un océan plus sain.

A volunteer with the Nature Conservancy grabs a handful of eelgrass flowering shoots to gather seeds for the world’s largest seagrass restoration project.

With so much going for it, why aren’t dive tourists flocking to explore seagrass meadows? I’ve encountered toothy sharks and crocodiles, lovable dugongs and turtles, amazing seahorses and octopuses, strange lumpfish and horseshoe crabs, and many others among seagrass.

Back in Florida, a single bonnethead shark eventually swam up to my camera, took a quick look and disappeared. I had one usable photograph after nine days of patience. I never found my key. A crab probably dragged it down one of the many nearby holes — perhaps it was a bit of revenge from nature. If we let ourselves destroy too much of nature, I worry about its revenge against all of us. Let’s do what we can to protect our seagrass — that’s one key we don’t want to lose.


1. Leigh SC, Papastamatiou YP, German DP. Seagrass digestion by a notorious ‘carnivore.’ Proceedings of the Royal Society B. 2018; 285(1886). doi.org/10.1098/rspb.2018.1583. royalsocietypublishing.org/doi/full/10.1098/rspb.2018.1583

2. Importance des herbiers marins. Florida Fish and Wildlife Conservation Commission (Commission de conservation de la faune et de la flore de Floride). myfwc.com/research/habitat/seagrasses/information/importance

3. Salinas C, Duarte CM, Lavery PS, et al. Seagrass losses since mid-20th century fueled CO2 provenant des stocks de carbone du sol. Biologie du changement global 2020; 26(9):4772–4784. doi.org/10.1111/gcb.15204. onlinelibrary.wiley.com/doi/full/10.1111/gcb.15204

En savoir plus

Regardez cette vidéo pour en savoir plus sur le rôle des herbiers marins dans le changement climatique.

© Alert Diver — Q3/Q4 2021